Any inverter used to feed electrical power from a generation unit into an AC grid, especially those designed as three-phase inverters, should comprise an EMC filter for interference suppression at the output, and to comply with all directives regarding electromagnetic compatibility (EMC). The design of an EMC filter is typically based on a combination of X and Y capacitors providing EMC filter capacitances that are connected between the individual output terminals of the inverter and between the output terminals and ground, respectively. Since the X capacitors have to be provided for each individual phase, the total X capacitance is three times higher in a three-phase inverter than in a single-phase inverter. Directives also require that, whenever an inverter is detached from an AC grid, the charge stored in the EMC filter capacitances must fall below a level that would otherwise be unsafe for humans within a specific period of time. To meet this requirement, the X capacitors at the output of the inverter are usually connected in parallel with discharge resistors that serve to discharge both the X and Y capacitors in the requisite time whenever the inverter is detached from the AC grid. The electrical energy stored in the EMC filter capacitances is then converted into heat. This conversion into heat also occurs while the inverter is running, as long as a voltage is applied to the X capacitors. A power loss will therefore result even if the inverter is not feeding electrical energy into the AC grid. Moreover, this power loss increases with the sum of the EMC filter capacitances at the output of the inverter due to the amount of current needed to flow through the discharge resistors in order to sufficiently discharge all EMC filter capacitances within the requisite time.
For inverters that supply electrical energy to an AC grid from a generation unit that is not permanently available (e.g., a photovoltaic system), it is known that at least a portion of the control and/or communication systems of the inverter is supplied with electrical power from the AC grid rather than the generation unit. Supplying electrical power from the AC grid may also be viewed as an alternative to supplying electrical power from the generation unit. To the end of supplying electrical power from the AC grid, one known inverter comprises a power supply unit that is supplied with a voltage applied to the output of the inverter by the AC grid. This voltage is also present at an EMC filter capacitance at the output of the inverter to which a discharge resistor is connected in parallel.
International Patent Application Publication WO 2009/014522A1 discloses a power system that combines a power source having a DC output with an AC supply from the AC grid to provide AC power to customer's loads and DC power to various DC auxiliary loads. The DC output of the DC power source is connected in steady-state to the DC input of a converter/bi-directional inverter for conversion therein to AC for connection to the customer's loads and to any AC auxiliary loads. During start-up of the DC power source, an open isolation switch disconnects that DC power source from the bi-directional inverter. A start-up power supply selectively connects between the AC power grid and the bi-directional inverter and/or DC controllers to provide a supply of rectified DC power at the inverter DC input and to certain DC auxiliary loads. DC power is supplied to the auxiliary loads from the inverter DC input substantially continuously during start-up and steady state. This power system also includes a sinusoidal or LCL filter between the output of the bi-directional inverter and a grid connect switch.
There still is a need for a method of discharging an EMC filter capacitance and for an inverter having at least one EMC filter capacitor in which a continuous conversion of electrical power into heat by discharge resistors is prevented despite having the EMC filter capacitances discharged at a sufficiently fast rate.